Greg McElroy is sacked by defensive end Kendall Reyes in the 2nd half of the Jets' 27-17 loss to San Diego. Photo: Neil Miller

The Jets quarterback carousel took another unexpected spin Thursday.

The team discovered starting quarterback Greg McElroy had suffered a concussion in Sunday’s loss to the Chargers after he experienced headaches while lifting weights Thursday morning.

Jets coach Rex Ryan said McElroy won’t play Sunday against the Bills and Mark Sanchez, who McElroy replaced last week as the starter, is back in as the starting quarterback.

Ryan picked Sanchez over Tim Tebow, who has not started a game for the Jets.

“I’m going to start Mark Sanchez,” Ryan said. “The reason I’m starting Mark is we have two practices and one walkthrough to get ready for Buffalo. Mark has had success earlier in the season against Buffalo. He’s very familiar with them. That’s the reason I’m going with Mark.”

Ryan began his news conference by saying, “You’re not going to believe this one.”

He then described the scene of McElroy and trainer John Mellody coming to his office around 9:30 Thursday morning to tell him that McElroy had not told the trainers he was experiencing any symptoms of a concussion after Sunday’s 27-17 loss to the Chargers. McElroy was sacked 11 times in the game and suffered an abdominal injury.

McElroy had not undergone concussion tests when Ryan spoke to the media, but the coach was convinced McElroy could not play this week.

“I’m not going to play him,” Ryan said. “There’s no doubt. I don’t care what the tests say. We’re going to err on the side of caution.”

For Sanchez, this is one last shot at having a positive come out of a dismal 2012 season. Sanchez was benched after his five-turnover performance two weeks ago against the Titans. He has 17 interceptions this season and 13 touchdowns.

This is another shot for Tebow to absorb. He voiced his frustration with being passed over last week for McElroy. Now, he was passed over again.

“Obviously, Tim’s not happy with that as you’d expect,” Ryan said.

Tebow did say that he understood the decision that the coaching staff made to go with Sanchez as the starter once again.

“I understand,” Tebow said. “I told coach I definitely understand. Obviously, you’re a competitor, you want to play, but I understood his decision. I just let him know, just like always, I’ll do whatever I can to help the team.”